The Awkward Alarm

A rather annoying little noisemaker.

Assembly suggestions

• When populating the board, it will be easiest to populate in the following order (don't forget the components on the bottom!):
  • MMDT3946
  • Resistors
  • Capacitors except tantalums and those outlined in red in the images to the right; see note below
  • MOSFET
  • Diode
  • Microcontroller
  • H-bridge
  • Tantalum capacitors
  • Potentiometer
  • Power leads
  • Reset switch
  • Inductors
  • Speaker leads
  
  
• Be sure to skip installing the capacitors highlighted in red until after you have programmed the microcontroller. Leaving the 470pF capacitor disconnected ensures that the MOSFET does not turn on spuriously, and the bypass capacitors on the reset line interfere with the programmer's communicating with the device.
  
  
• The board's silkscreen indicates an 82uH inductor; this value is incorrect, and should be 33uH.
  
  
• The programming leads should be connected to the SiLabs USB Debug Adapter as follows:
  • C2CK/RST (the pin next to "PRG" on the silkscreen) goes to pin 7
  • C2D/P2.7 (the pin next to the "=" on the silkscreen) goes to pin 4
  • GND (board ground) goes to pin 9
  • the debug adapter can provide VDD from pin 10, or you can just connect your own power supply to the board

Design Files

The design tarball includes all design files, including code, board and schematic (in Eagle format), gerbers, makefiles, et cetera. As should be clear from the Makefile, building the code requires SDCC. It should also be possible to use the SiLabs IDE as long as you use SDCC with it, though I haven't tested compilation in the IDE.

I have used both ec2drv and the SiLabs IDE to program the microcontroller on the board; both should work without an issue. See above for information on connecting the board to the debug adapter. You should also be able to program using the much cheaper SiLabs ToolStick, though I haven't tried it.

BOM

Note: Any appropriate panel mount potentiometer or momentary switch can be substituted for the corresponding components below. The speakers listed below are only a suggestion; this circuit can drive most 4 or 8Ω speakers, but be sure the speaker's power rating is sufficient.

Component	Value	Quantity	Size/Type	Digi-Key P/N
Resistor	220Ω	1	0402	311-220JRCT-ND
Resistor	3.3kΩ	2	0402	311-3.3KJRCT-ND
Resistor	15kΩ	1	0402	311-15KJRCT-ND
Resistor	68kΩ	2	0402	311-68KJRCT-ND
Potentiometer	50kΩ	1	6mm	CT6EP503-ND
Capacitor	470pF	2	0402 X7R	PCC471BQCT-ND
Capacitor	0.1uF	10	0402 Y5V	490-1327-1-ND
Capacitor	4.7uF	3	0402 X5R	PCC2475CT-ND
Capacitor	10uF	5	0805 Y5V	445-3458-1-ND
Capacitor	100uF	2	7343-31 Tantalum	718-1057-1-ND
Inductor	33uH	1	DR125	513-1528-1-ND
Inductor	47uH	1	NR6028 or DR73	587-2104-1-ND
Diode	SS12	1	SMA Schottky	SS12FSCT-ND
MOSFET	FDT457N	1	SOT-223 NMOS	FDT457NCT-ND
Dual BJT	MMDT3946	1	SOT-363 NPN/PNP	MMDT3946-FDICT-ND
H-Bridge	SI9986	1	8-SOIC	SI9986DY-T1-E3CT-ND
Microcontroller	C8051F410	1	32-LQFP	336-1318-ND
Switch	SPST (NO)	1	5mm	CKN9102-ND
Speaker (opt 1)	4W, 8Ω	1	40mm x 40mm	102-1305-ND
Speaker (opt 2)	2W, 8Ω	1	28mm round	102-1559-ND
Case (opt 1)	RL6105	1	80mmx60mmx30mm	HM1068-ND
Case (opt 2)	RL6115	1	80mmx60mmx40mm	HM1069-ND
Battery case	SBH-431-1A	1	3xAAA	SBH-431-1A-ND